The present invention relates to hydrostatic vehicle power steering systems, and more particularly, to such systems for use on vehicles which typically travel at relatively higher speeds, including but not limited to "on-highway" vehicles, such as automobiles and light trucks, as well as "high speed" tractors, i.e., those intended to travel at speeds in excess of about 30 mph. For simplicity, both types of vehicles will hereinafter be included within either of the terms "on-highway" and "high speed".
The typical vehicle power steering system has been one of two general types, depending upon the vehicle application. Traditional on-highway vehicles have utilized a mechanical steering gear (e.g., rack and pinion) with hydraulic power assist. In this type of system, there is a mechanical linkage between the steering wheel and the steered wheels, so that in the event of a loss of hydraulic power, the driver is still able to steer the vehicle. "Off-highway" vehicles have utilized a full fluid-linked steering system ("fluid-linked system") in which rotation of the steering wheel actuates a steering control unit ("SCU"), which is actually a valve controlling the flow of pressurized fluid from a pump, through the SCU, to a steering cylinder or other suitable device. The SCU conventionally includes a fluid meter which imparts follow-up movement to the valving, thus returning the valving to its neutral position after the desired amount of fluid has been directed by the SCU to the steering cylinder.
More recently, those skilled in the vehicle steering art have proposed hybrid mechanical-electrical systems, having no hydraulics, in which there is the conventional mechanical connection between the steering wheel and the steered wheels, but with the mechanical input receiving an electric motor assist. The proposed systems would include sensors to sense input parameters such as steering wheel position and torque, with these inputs being transmitted to the vehicle microprocessor to generate the appropriate electrical signal to energize the electric motor.
The mechanical steering gear type of system, whether hydraulically assisted or electric motor assisted, has several disadvantages. In the conventional mechanical steering gear systems, there is a rigid steering column extending from the steering wheel to the steering gear. In the event of a front-end collision, the steering column is, unfortunately, in the perfect location, and at nearly the perfect angle, to be forced through the firewall and toward the driver. Therefore, complicated and expensive solutions are necessary to make the steering column collapsible, to at least reduce the likelihood of injury to the driver caused by the steering column. In addition, the presence of the steering column in the engine compartment presents substantial challenges to the vehicle designer in regard to the layout and location of the engine and the various engine accessories. As is understood by those skilled in the vehicle art, the designer has very little choice with regard to the location of the steering wheel, or the location of the mechanical steering gear, and it is necessary to maintain a straight, clear "path", between the steering wheel and the steering gear, for the steering column.
Another disadvantage of the conventional mechanical steering gear type of system has more recently become apparent. As the market for many cars and light trucks has become more "global" in nature, there has arisen a need for the vehicle manufacturers to offer both "left-hand drive" and "right-hand drive" versions of each particular model. The need to effectively "reverse" the steering system, in order to convert a normally left-hand drive vehicle to right-hand drive, or vice versa, may necessitate that some of the steering system components (e.g., the rack and pinion assembly) be duplicated, in the sense of having two different part numbers to inventory, wherein the parts are substantially identical, but reversed. The change between left-hand and right-hand drives is also likely to result in some major rearrangements within the engine compartment, in order to accommodate the steering column on the opposite side of the engine from its normal, left-hand location. This can be an especially difficult rearrangement in the case of transverse engine vehicles.
Greater system flexibility, in terms of system layout, can be achieved utilizing the full fluid-linked type of system which has traditionally been used on off-highway vehicles. In other words, each component of a fluid-linked system could be used for either left- or right-hand drive, and the only connection between the SCU and the actuator is hoses, which greatly simplifies the task of laying out the engine compartment. However, several characteristics commonly found in such systems have seemingly made the fluid-linked systems ill-suited for on-highway vehicles. First, the fluid-linked systems have typically had insufficient "load reaction" or road feel, and thus are normally not acceptable to the driver. Second, there has generally been excessive "neutral dead band", such that the steering control at relatively higher vehicle speeds is unacceptable. Third, the "registry", or position correlation, between the steering wheel and the steered wheels has typically not been maintained sufficiently, especially at the end of travel, to satisfy the on-highway driver.